Is there a reason you have the middle show for HDD vs the early show that would allow you to do both the HDD and the MK Fireworks from inside the park?
With a 6:15 PM show the best you could do is maybe watch the high Fireworks from the FW boat dock, or from the first boat if you happen to have Category 1 seating any you are seated very near the back of the main floor.
As
@scrappinginontario indicated the show will not end until right at 8:15 PM.
Dave